The measurement of blood pressure within the human heart and its vasculature provides critical information regarding the organ's function. Many methods and techniques have been developed to give physicians the ability to monitor heart function to properly diagnose and treat various diseases and medical conditions. For example, a sensor placed within the chambers of the heart can be used to record variations in blood pressure based on physical changes to a mechanical element within the sensor. This information is then transferred through a wire from the sensor to an extracorporeal device that is capable of translating the data from the sensor into a measurable value that can be displayed. The drawback of this type of sensor is that there must be a wired connection between the sensor and the extracorporeal device, thus limiting its use to acute settings.
Many types of wireless sensors have been proposed that would allow implantation of the device into the body. Then, through the appropriate coupling means, pressure reading can be made over longer periods of interest. The primary limitation to these type of sensors is that the fabrication methods used to manufacture them do not provide sufficient miniaturization to allow them to be introduced and implanted into the heart using non-surgical, catheter-based techniques while maintaining the ability to communicate wirelessly with external electronics.
An implantable sensor of this type must be assembled using the materials and fabrication methods that ensure appropriate biocompatibility and long term mechanical and electrical durability.
One method of manufacturing a sensor capable of measuring pressure is to use a capacitor that is assembled such that one of the capacitive plates will be displaced with respect to the other as a result of exposure to externally applied stress. This displacement will result in a change in the capacitance that is proportional to the applied stress. Various patents describe the fabrication and use of capacitor-based pressure sensors. The primary limitation of many of these inventions is that the techniques used to fabricate the sensors do not lend themselves to the miniaturization necessary for it to be configured as an implantable medical device while maintaining the capability of communicating wirelessly with external electronics.
The fabrication methodologies that have been developed in the field of Micro-Electro-Mechanical Systems (“MEMS”), however, do specifically provide the means for assembling miniaturized sensors capable of measuring a variety of properties including pressure. MEMS devices as described in prior patents traditionally use silicon as a substrate for construction of miniature electrical or mechanical structures.
A number of patents detail pressure sensors (some capacitive in nature, some manufactured using MEMS based fabrication methods) that are specifically designed for implantation into the human body. These sensors suffer from many of the limitations already mentioned, with the additional concerns that they require either the addition of a power source to operate the device or the need for a physical connection to a device capable of translating the sensor output into a meaningful display of a physiologic parameter.
To overcome the two problems of power and physical connection, the concept of a externally modulated LC circuit has been applied to development of implantable pressure sensors. Of a number of patents that describe a sensor design of this nature, U.S. Pat. No. 6,113,553 to Chubbuck is a representative example. The Chubbuck patent demonstrates how a combination of a pressure sensitive capacitor placed in series with an inductor coil provides the basis for a wireless, un-powered pressure sensor that is suitable for implantation into the human body. Construction of an LC circuit in which variations of resonant frequency correlate to changes in measured pressure and in which these variations can be detected remotely through the use of electromagnetic coupling are further described in U.S. Pat. Nos. 6,111,520 and 6,278,379, both to Allen et al., incorporated herein by reference.
The device described in the Chubbuck patent is large, thus requiring surgical implantation and thereby limiting its applicability to areas that are easily accessible to surgery (e.g., the skull).
Thus, the need exists for a miniature, biocompatible, wireless, un-powered, hermetic pressure sensor that can be delivered into the heart or the vasculature using a small diameter catheter.